1. Field of the Invention
The invention relates to a battery system having a high-power battery and a high-capacity battery electrically connected in parallel to each other, and a control method thereof.
2. Description of the Related Art
Japanese Patent Application Publication No. 11-332023 (JP 11-332023 A) and Japanese Patent Application Publication No. 2008-041620 (JP 2008-041620 A) describe a system in which a high-power battery and a high-capacity battery are electrically connected in parallel to each other. The high-power battery can be charged and discharged with a larger current than the high-capacity battery, while the high-capacity battery has a higher storage capacity than the high-power battery.
In such a system having a high-power battery and a high-capacity battery electrically connected to each other in parallel, current (circulating current) sometimes flows from the high-power battery to the high-capacity battery. In order to prevent this circulating current, it is conceivable to keep the voltage of the high-capacity battery at a higher level than the voltage of the high-power battery.
However, when the voltage of the high-capacity battery is kept at a higher level than the voltage of the high-power battery, the available range of state-of-charge (SOC) for control of charging and discharging of the high-capacity battery may be restricted. In other words, it may become difficult to efficiently use electrical energy stored in the high-capacity battery.